The present invention relates to a method and apparatus for receiving communication signals. More particularly, the invention concerns channel estimation, impulse response shortening and symbol synchronization in an OFDM communications system.
In Orthogonal Frequency Division Multiplexing (OFDM), information is transmitted using a plurality of sub-carriers whose frequencies are uniformly spaced in a given bandwidth. In an OFDM communications system, generally speaking, the sequence of information source bits is mapped onto a set of N parallel sequences of complex numbers chosen from finite alphabets. The alphabets from which the complex numbers are chosen may be QAM alphabets or PSK alphabets; different alphabets may be used on different sequences. The mapping of the information source bits onto the N parallel sequences of complex numbers will generally involve some form of forward error correction coding and permutation, in order to make the system robust against channel impairments such as noise and frequency-selectivity.
Each sequence of complex numbers is then transmitted on a separate sub-carrier using Quadrature-Amplitude-Modulation of a rectangular pulse of length T seconds. If the pulses transmitted on the separate sub-carriers are mutually aligned, the sub-carriers are uniformly spaced 1/T Hz apart and the channel response is constant over the entire frequency band, then each received sub-carrier can be individually demodulated without any inter-carrier interference or inter-symbol interference. If the channel response is not constant, then choosing T to be much larger than the delay spread of the channel will reduce the inter-carrier interference and inter-symbol interference. However, for a given total bandwidth, a large value of T will result in a large value of N, and thus in a system with large complexity.
For channels whose delay-spread is limited, the preferred approach is to increase the length of the rectangular pulse with a cyclic guard interval of Tg seconds, which is greater than the delay spread. This allows the receiver to identify some T-second-long portion of each received pulse that is free of inter-carrier interference and inter-symbol interference, correct for amplitude and phase changes caused by the channel, and demodulate it.
The use of a cyclically extended guard portion allows low-complexity demodulation of the received signal, without having to worry about inter-sub-carrier interference or inter-symbol interference. To realize these benefits, the length of the cyclically extended guard portion must be greater than the length of the effective overall impulse response of the cascade of the transmitter filters, propagation channel, and the receiver filters. The price being paid is the reduction in throughput by a factor T/(T+Tg) and a reduction in power efficiency by approximately the same factor.
In some scenarios, the effective overall impulse response can be very long due the delay-spread of propagation channel being very large or the impulse response of the transmit/receive filter being very long. In these scenarios, choosing Tg to be greater than the length of the overall impulse response will result in the throughput and efficiency of the system being extremely poor. Some proposals suggest the use of a sub-optimal receiver filter so as to achieve a short overall impulse response of a desired length—the receiver filter is sub-optimal in the sense that the noise at the output may be correlated, and the degree of correlation is a measure of the sub-optimality of the receiver filter.
Several methods have been previously proposed to separately solve the problems of channel estimation, impulse response shortening, and symbol synchronization in OFDM communication systems. See for example: J. S. Chow, J. M. Cioffi, and J. A. Bingham, “Equalizer Training Algorithms for Multicarrier Modulation Systems”, Proceedings of 1993 International Conference on Communications, Geneva, Switzerland, May 1993, pp 761-765; D. Pal, J. M. Cioffi, and G. lyengar, “A new method of channel shortening with applications to DMT systems”, International Conference on Communications, pp. 763-768, June 1998; and P. J. W. Melsa, R. C. Younce, and C. E. Rohrs, “Impulse response shortening for discrete multitone receivers”, IEEE Trans. Commun., pp. 1662-1672, December 1996.
Some of the disadvantages of previous methods are that longer training signals are needed because the training signal must have portions dedicated to each of these tasks. Accordingly, an OFDM system in which all three problems could be jointly solved using the same training signal is desirable as it that would to increase system throughput.